Seawater has a higher concentration of ¹⁸O than freshwater. When three of the oldest known ancient whales were tested for ¹⁸O, the ratio was similar to amounts found in modern river dolphins that live in freshwater. The fourth younger Archaeoceti, Indocetus, had higher concentrations of ¹⁸O.

This tells us that ancestors of cetaceans had to drink freshwater to survive and could not stray far from coastlines. An example is Ambulocetus, whose skeletal remains are found in saltwater deposits, but who's teeth tell us it drank freshwater. However, within a short period of geologic time (4 million years), ancient cetaceans were able to specialize their kidneys in order to drink saltwater.(Monastersky, 1996)